In recent years, in rapidly developing mobile communication system (for example, a Personal Handy Phone System, hereinafter referred to as PHS), a PDMA (Path Division Multiple Access) system has been proposed, in which an identical time slot of an identical frequency is spatially divided for an efficient use of radio wave frequencies, and mobile terminals of a plurality of users can establish path multiple access to a mobile base system. In the PDMA system, a signal from a mobile terminal of each user is separately extracted by well-known adaptive array processing. The PDMA system is also referred to as an SDMA (Spatial Division Multiple Access) system.
FIG. 8 illustrates arrangements of channels in various communication systems: a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system and a spatial division multiple access (SDMA) system.
With reference first to FIG. 8, the FDMA, TDMA and SDMA systems are briefly described. FIG. 8(a) illustrates the FDMA system, in which analog signals of users 1 to 4 are frequency-divided and transmitted with radio waves having different frequencies f1 to f4 so that the signals of each user 1 to 4 are separated by a frequency filter.
In the TDMA system shown in FIG. 8(b), digitized signals of each user are time-divided and transmitted with radio waves having different frequencies f1 to f4 at every certain time (time slot) so that the signals of each user are separated by a frequency filter as well as time synchronization between a base station and a mobile terminal of each user.
On the other hand, recently, portable telephone sets have been widely used, and the SDMA system is proposed to enhance efficient use of radio wave frequency. This SDMA system is employed for spatially dividing one time slot at the same frequency and transmitting data of a plurality of users, as shown in FIG. 8(c). In this SDMA system, signals of each user are separated, using a frequency filter, time synchronization between a base station and a mobile terminal of each user, and a mutual interference eliminator such as an adaptive array.
FIG. 9 is a schematic block diagram showing a configuration of a transmission/reception system 2000 of a conventional SDMA base station.
In the configuration shown in FIG. 9, four antennas #1 to #4 are provided for identifying users PS1 and PS2.
In a receiving operation, outputs from the antennas are supplied to an RF circuit 2101. In RF circuit 2101, outputs are amplified through a receiving amplifier, and frequency-converted with a local oscillation signal. Thereafter, unnecessary frequency signals are removed through a filter, A/D converted, and supplied to a digital signal processor 2102 as digital signals.
Digital signal processor 2102 is provided with a channel allocation reference calculator 2103, a channel allocator 2104 and an adaptive array 2100. Channel allocation reference calculator 2103 calculates in advance whether or not the adaptive array can separate signals received from two users. In response to the result of this calculation, channel allocator 2104 supplies channel allocation information including user information for selecting a frequency and time to adaptive array 2100. Adaptive array 2100 performs operation, in real time, of the signals from the four antennas #1 to #4 based on the channel allocation information, and assigns weights thereto. Thus, only a signal from a specific user is separated.
[Configuration of Adaptive Array Antenna]
FIG. 10 is a block diagram showing a configuration of a transmission/reception unit 2100a corresponding to one user among adaptive array 2100. In the example shown in FIG. 10, n input ports 2020-1 to 2020-n are provided for extracting a signal of a desired user from input signals including a plurality of user signals.
Signals input to each input port 2020-1 to 2020-n are supplied to a weight vector control unit 2011 and multipliers 2012-1 to 2012-n through switching circuits 2010-1 to 2010-n.
Weight vector control unit 2011 calculates weight vectors w1i to wni, using the input signals, a unique word signal stored in advance in a memory 2014 and corresponding to the signal from the specific user as well as an output of an adder 2013. Here, the subscript i indicates a weight vector used for transmission/reception to/from an ith user.
Multipliers 2012-1 to 2012-n multiply the input signals from each input port 2020-1 to 2020-n by weight vectors w1i to wni respectively, and supply the results to adder 2013. Adder 2013 adds up the output signals from multipliers 2012-1 to 2012-n, and outputs the sum as a reception signal SRX(t), which is also supplied to weight vector control unit 2011.
Transmission/reception unit 2100a further includes multipliers 2015-1 to 2015-n, which receive an output signal STX(t) from an adaptive array radio base station, multiply the same by weight vectors w1i to wni supplied from weight vector control unit 2011 respectively, and output the results. The outputs of multipliers 2015-1 to 2015-n are supplied to switching circuits 2010-1 to 2010-n respectively. In other words, switching circuits 2010-1 to 2010-n, when receiving the signals, supply the signals received from input ports 2020-1 to 2020-n to a signal receiving unit 1R and, when transmitting the same, supply signals from a signal transmission unit 1T to input/output ports 2020-1 to 2020-n.
[Operation Principle of Adaptive Array]
The operation principle of transmission/reception unit 2100a shown in FIG. 10 is now briefly described.
In the following, for the sake of simplicity, it is assumed that the number of antenna elements is four and the number of users PS simultaneously in communication is two. In this case, signals supplied from each antenna to receiving unit 1R are expressed as follows:RX1(t)=h11Srx1(t)+h12Srx2(t)+n1(t)  (1)RX2(t)=h21Srx1(t)+h22Srx2(t)+n2(t)  (2)RX3(t)=h31Srx1(t)+h32Srx2(t)+n3(t)  (3)RX4(t)=h41Srx1(t)+h42Srx2(t)+n4(t)  (4)
where signal RXj(t) represents a reception signal of a jth (j=1, 2, 3, 4) antenna, and signal Srx1(t) represents a signal transmitted from an ith (i=1, 2) user.
Further, coefficient hji represents a complex coefficient of a signal from the ith user, received at the jth antenna, and nj(t) represents noise included in a jth reception signal.
The above equations (1) to (4) are expressed in a vector form as follows:X(t)=H1Srx1(t)+H2Srx2(t)+N(t)  (5)X(t)=[RX1(t), RX2(t) . . . , RXn(t)]T  (6)Hi=[h1i, h2i, . . . , hni]T, (i=1, 2)  (7)N(t)=[n1(t), n2(t), . . . , nn(t)]T  (8)
Note that [ . . . ]T represents transposition of [ . . . ] in equations (6) to (8).
In the equations, X(t) represents an input signal vector, H1 represents a reception signal coefficient vector of the ith user, and N(t) represents a noise vector respectively.
As shown in FIG. 10, an adaptive array antenna outputs, as a reception signal SRX(t), the signal synthesized by multiplying the input signals from respective antennas by weight coefficients w1i to wni. Note that the number n of antennas is set to four.
In order to extract a signal Srx1(t) transmitted from a first user, for example, after the aforementioned preparation, the adaptive array operates in the following manner.
An output signal y1(t) from adaptive array 2100 can be expressed as follows, by multiplying the input signal vector X(t) by a weight vector W1.y1(t)=X(t)W1T  (9)W1=[w11, w21, w31, w41]T  (10)
Here, weight vector W1 has a weight coefficient wji (j=1, 2, 3, 4) multiplied by jth input signal RXj(t) as an element.
Here, input signal vector X(t) expressed in the equation (5) is substituted in y1(t) expressed in the equation (9).y1(t)=H1W1TSrx1(t)+H2W1TSrx2(t)+N(t)W1T  (11)
When adaptive array 2100 ideally operates, weight vector control unit 2011 sequentially controls weight vector W1 by a well-known method so as to satisfy the following simultaneous equations.H1W1T=1  (12)H2W1T=0  (13)
When weight vector W1 is completely controlled to satisfy the equations (12) and (13), output signal y1(t) from adaptive array 2100 is finally expressed as follows.y1(t)=Srx1(t)+N1(t)  (14)N1(t)=n1(t)w11+n2(t)w21+n3(t)w31+n4(t)w41  (15)
In other words, for output signal y1(t), signal Srx1(t) transmitted from the first user out of two can be obtained.
Referring to FIG. 10, input signal STX(t) for adaptive array 2100 is supplied to transmission unit 1T in adaptive array 2100, and to one input of multipliers 2015-1, 2015-2, 2015-3, . . . , 2015-n. Weight vectors w1i, w2i, w3i, . . . , wni calculated by weight vector control unit 2011 based on the reception signals as described above are copied and applied to the other inputs of these multipliers respectively.
The input signals weighted by these multipliers are sent and transmitted to corresponding antennas #1, #2, #3, . . . , #n through corresponding switches 2010-1, 2010-2, 2010-3, . . . , 2010-n.
Here, the users PS1 and PS2 are identified in the following manner. A radio signal from a portable telephone set is transmitted with a frame structure. The radio signal from the portable telephone set roughly includes a preamble consisting of a signal series already-known to the radio base station as well as data (voice etc.) consisting of a signal series unknown to the same.
The signal series of the preamble includes a signal string of information for identifying whether or not one particular user is a desired user for the radio base station to establish communication with. Weight vector control unit 2011 of adaptive array radio base station 1 compares the unique word signal corresponding to user A fetched from memory 2014 with the received signal series, and performs weight vector control (decision of a weight coefficient) so as to extract a signal seeming to include a signal series corresponding to user PS1.
During an operation for establishing communication, information for identifying a terminal that requested a connection to the base station is communicated between the base station and the terminal.
For example in the PHS system, however, once the communication is established, generally, information for identifying a particular user is not contained in the unique word signal (UW signal) described above. Therefore, in principle, the base station cannot identify the terminal being in communication. This will apply not only to the PDMA system as described above but also to the conventional PHS system of TDMA, for example.
In the PHS system of the conventional TDMA system, it is known that, between adjacent base stations, users talking on the phone may be interchanged, or communication with a terminal connected with a particular base station may be interrupted by a radio wave from other terminal. Such poor communication is called “SWAP”.
FIG. 11 is a conceptual illustration showing one mode of swap in the PHS system of such conventional TDMA system.
FIG. 11 shows an interchange of signals communicated between a terminal PS1 of a user 1 in communication with a base station CS1 and a terminal PS2 of a user 2 in communication with an adjacent base station CS2.
FIG. 12 is a conceptual illustration showing another mode of swap in the PHS system of such conventional TDMA system.
In FIG. 12, communication of terminal PS1 of user 1 in communication with base station CS1 with a signal PS1 has been interrupted by a signal PS2 from terminal PS2 of user 2 in communication with adjacent base station CS2.
If swap as described above occurs, a signal corresponding to other terminal will be heard as a noise at the terminal of the user originally in communication, because signals PS1 and PS2 are scrambled differently.
Moreover, in a mobile communication system in accordance with the PDMA system as described above, a reception timing (also referred to as a synchronous position) when a signal transmitted from each mobile terminal arrives at a radio base station will vary by various factors such as change of a distance from a terminal to a base station due to the travel of the terminal, and variation in a property of radio wave propagating path. When mobile terminals of a plurality of users establish path multiple access to an identical time slot in the mobile communication system in accordance with the PDMA system, reception timings of reception signals from respective mobile terminals may vary and come closer to each other, because of the above-described reasons. Possibly, temporal relation may be inverted.
If the reception timings are too close to each other, a correlation value between the reception signals from the plurality of mobile terminals will be high, resulting in lower accuracy in signal extraction per user by adaptive array processing. This will also lead to deterioration of communication property for each user. Here, in the PHS system, as described above, the reception signal from each mobile terminal includes a reference signal (a unique word signal) section consisting of an already-known bit string common to all users for each frame. Therefore, if the reception timings of the reception signals from the mobile terminals of the plurality of users should coincide, the reference signal sections of the reception signals will overlap, and each user cannot be identified separately, thus causing an interference between users (SWAP as described above).
In addition, in the mobile communication system in accordance with the PDMA system, if the number of users establishing multiple access to each time slot increases, that is, degree of path multiplicity rises, a transmission timing interval in each slot will inevitably be narrowed. As a result, the reception timings may come closer to each other or cross. In such a case, as described above, it is more likely that communication property may be lowered, or interference between users may be caused.
FIG. 13 is a conceptual illustration showing one mode of swap in the PHS system in accordance with the PDMA system.
In FIG. 13, signals communicated between terminal PS1 of user 1 in communication with base station CS1 through one path and terminal PS2 of user 2 in communication with the same through another path have been interchanged.
FIG. 14 is a conceptual illustration showing another mode of swap in the PHS system in accordance with the PDMA system.
In FIG. 14, communication of terminal PS1 of user 1 in communication with base station CS1 with signal PS1 through one path has been interrupted by signal PS2 from terminal PS2 of user 2 in communication with the same through another path.
In this case as well, interference will considerably lower communication quality, as in the conventional PHS system.